The present invention generally relates to boilers or steam generators for electric power generation and, in particular, to a new and useful design of a sliding pressure once-through boiler using both single-lead and multi-lead ribbed tubing.
In the power plant field, once-through boilers have been used since 1926. The design of the once-through boiler includes provisions for sliding pressure operation to accommodate supercritical steam pressure. As shown in FIG. 1, a boiler feed pump 8 for the system 10 provides the entire driving head to force the water through an economizer 11, an evaporator 12, and a superheater 14 which can be used in conjunction with a separator 13. Water is continuously evaporated to dryness and then superheated without any steam-water separation. This circulation method is applicable to all operating pressures, i.e. supercritical (greater than 3208 psia) and subcritical (less than 3208 psia). Typically, the system 10 uses spiral furnace circuitry for the evaporator 12 because a vertical tube design is more sensitive to upsets and nonuniform tube-to-tube heating. However, for start-up and low load operation, special by-pass systems are still needed.
To accommodate start-up and low load operating conditions, once-through boiler designs with superimposed recirculation systems 10a and 10b have been used and are illustrated in FIGS. 2 and 3. These recirculation systems permit partial recirculation of fluid to the furnace walls in order to increase the fluid velocity in the evaporator tubes by incorporating circulation pumps 15 and orifices 16. The design in many applications allows the furnace 12 to remain at constant pressure, typically supercritical pressure, and utilizes a separator or flash tank 13 for reducing the superheater pressure to subcritical pressures at start-up and low loads. These types of once-through boiler systems 10a and 10b typically utilize a vertical furnace tube evaporator design.
Once-through boiler designs that utilize both spiral and vertical tube furnace evaporators have been sold by many boiler manufacturers. These designs have been developed for either supercritical or subcritical steam pressures. However, a vertical tube once-through boiler for sliding pressure applications has recently been put into service. The sliding pressure operation of this vertical tube once-through boiler is restricted to approximately 40% minimum load due to the flow requirements of the evaporator. Except for certain fuels and sizes of spiral-tube furnaces, a spiral tube furnace does not have this restriction. For these few exceptions, a higher than desired minimum flow results due to the flow requirements of the evaporator. The spiral tube furnace permits greater freedom in matching the tube diameter and mass velocity of the furnace to ensure tube cooling and flow stability in the parallel furnace evaporator tubes. It also allows each tube of the furnace to run through all of the various heat zones in the combustion chamber, so that differences in total heat input between tubes will be kept to a minimum.
The development of a vertical tube sliding pressure once-through boiler is needed due to the higher cost of the spiral furnace design when compared to a vertical furnace design. The construction of a forced circulation once-through boiler requires the use of a very large number of parallel tubes, welded together to form membrane panels. A fundamental requirement for membrane wall integrity is uniform fluid and metal temperature in all tubes at each furnace level. Until now, the major problem with the vertical tube design was due to the large heating difference between individual tubes in the furnace. In vertical tube furnaces, the heating difference between tubes is approximately 2.5 times as great as in a spiral tube furnace design. Average mass velocities of 1,500,000 to 2,000,000 lb/hr-ft.sup.2 are typical velocities, used in current once-through boiler designs. These mass velocities when subjected to typical peripheral furnace heat absorption variations (which can be 35% or more than average), result in a velocity variation that decreases in magnitude. This trend is called the once-through characteristic of a boiler tube. In the once-through mode, the velocity change due to an increase in heat is negative as shown in FIG. 4. If excessive heat input is applied to a single tube, a reduction in fluid mass velocity occurs in that tube, causing an additional increase in the outlet temperature of the fluid in the tube.
If the furnace tubes are operated with reduced mass velocities, the result with respect to any single tube that is exposed to excessive heat is an increase in the mass velocity. This type of change in mass velocity is referred to as the natural circulation characteristic. To be able to use lower mass velocities for the furnace design of a vertical tube once-through boiler requires the use of ribbed tubes in the burner zone to avoid departure from nucleate boiling (DNB).
Single-lead ribbed (SLR) tubes have been employed by The Babcock & Wilcox Company (B&W), assignee of the present invention, in once-through, vertical tube, subcritical pressure boilers. Multi-lead ribbed (MLR), tubes have been employed by B&W in some applications to once-through, vertical tube, supercritical pressure boilers, and to once-through, spiral-tube boilers operating at both sub- and supercritical pressures. Examples of these SLR and MLR tube geometries are shown in FIGS. 6-8 herein. U.S. Pat. Nos. 3,088,494 and 3,289,451 to Koch, et al. disclose, respectively, ribbed vapor generating tubes for sub-critical pressure vapor generators, and a method and apparatus for forming internal helical ribbing in a tube of the type disclosed in U.S. Pat. No. 3,088,494.
For these types of ribbed tubes, the heat transfer characteristics of the tubes are extremely good even for low mass velocities of fluid through the tubes. Generally SLR tubes permit higher heat fluxes than MLR tubes for the same mass velocity. The heat transfer performance of the SLR tube has been documented in the open literature. See, for example:
(1) "The Effects of Nucleate Boiling Versus Film Boiling on Heat Transfer in Power Boiler Tubes", H.S. Swenson, J.R. Carver, G. Szoeke, Journal of Engineering for Power, Trans. ASME, Oct. 1962, pp. 365-71;
(2) "Flow Boiling Crisis in Grooved Boiler-Tubes", K. Nishikawa, T. Fujii, S. Yoshida and M. Ohno, Proceedings of the Fifth International Heat Transfer Conference, Vol. IV, 1974, pp. 270-74; and
(3) Steam: its generation and use, 40th ed., Copyright.COPYRGT. 1992, The Babcock & Wilcox Company.
The heat transfer performance of MLR tubes has also been documented in the open literature. See, for example, reference (3) above, as well as the following references:
(4) "Critical Heat Flux in Inclined and Vertical Smooth and Ribbed Tubes", G.W. Watson, R.A. Lee, and M. Wiener, Fifth International Heat Transfer Conference, Vol. IV, 1974;
(5) "Latest Developments in Natural Circulation Boiler Design", M. Wiener, Proceedings of the American Power Conference, Apr. 18-20, 1977;
(6) "Elements of Two-Phase Flow in Fossil Boilers", J.B. Kitto and M.J. Albrecht, presented to the NATO Advanced Study Institute on Thermal-Hydraulic Fundamentals and Design of Two-Phase Flow Heat Exchangers, Porto, Portugal, Jul. 6-16, 1987.
(7) "Fossil-Fuel-Fired boilers: Fundamentals and Elements", J.B. Kitto and M.J. Albrecht, Chapter 6 of Boilers, Evaporators and Condensers, pp. 179-275, John Wiley and Sons, Inc.; and
(8) "Heat Transfer Characteristics of Rifled Tubes in the Near Critical Pressure Region", Makio Iwabuchi, Mikio Tateiwa, Hisao Haneda, Proceedings of the 7th International Heat Transfer Conference, Vol. 5, 1982, pp. 313-18.
In a once-through, vertical tube, sliding pressure boiler design the use of the MLR tubes described above is not sufficient for developing the most optimized furnace evaporator design. The main reason is that higher mass velocities must be used to avoid departure from nucleate boiling (DNB), and thus the benefit of the natural circulation characteristic is lost. In addition, a higher mass velocity requires larger feed pumps and uses more power, which is an economic disadvantage. Heat transfer problems at or very near the critical pressure point (3208 psia) exist with the use of MLR tubes. As described in Reference 8 above, at or near the critical pressure point the effect of the swirl in the MLR tubes diminishes, due to the small density difference between steam and water, causing critical heat flux (CHF) conditions to appear at higher mass velocities than in SLR tubes. The CHF in tubes causes excessive metal temperatures which must be avoided. This problem causes difficulty in designing a furnace evaporator for sliding pressure operation at low mass velocities. For sliding pressure operation, a certain load point will exist where the heat flux applied to the tube will be high enough that the heat transfer through the MLR tube wall into the fluid therein will not be sufficient and elevated tube temperatures will exist. The possibility of furnace tube failure could occur.
It is thus apparent that a new design for a once-through sliding pressure vapor generator is needed to overcome the above-described disadvantages. In particular, a once-through, vertical tube, sliding pressure boiler or steam generator is needed in the field due to the higher cost of spiral furnace designs when compared to a vertical tube design.